Within the modern technique of chip removing machining, tools having replaceable wear parts in the form of cutting inserts are mostly used, which are mounted in form-fitted seatings or insert seats in different basic bodies that are suitable for, e.g., milling, drilling and turning, respectively. Generally, the cutting inserts are manufactured from a material that is considerably harder and more wear-resistant than the material of the basic bodies, such as steel, aluminum and the like. For the applications and machining methods that through the years have been developed to efficiently machine myriads of details of most varying materials, in particular metallic materials, there have been provided cutting inserts of a range of different materials that have their individual advantages and disadvantages for the various applications. A dominant material of the cutting inserts is cemented carbide, which is an overall denomination of such powder metallurgical products that are built up from a number of hard carbides (wolfram carbide, titanium carbide, tantalum carbide, niobium carbide) in a binder metal (usually cobalt). Cutting inserts formed of cemented carbide have generally good properties in respect of hardness in combination with a certain toughness (imparted by the binder metal), which among other things, means that the cutting inserts may be constructed fairly freely in a most varying geometrical shape.
However, for certain applications, cemented-carbide inserts are unsuitable, for instance, as a consequence of the limited hot hardness of the cemented carbide, i.e., the capability of the material to retain its hardness at high temperatures. During machining of certain types of metals, and under certain outer conditions, extreme heat is generated upon the chip removal, which tends to make cemented-carbide inserts less suitable for their purpose. However, in such cases, ceramic cutting inserts can often be used in a successful way. The ceramic material and the cemented carbide have, per se, certain common properties, but also differ from each other in several important respects. Thus, compared with cemented-carbide inserts, the ceramic cutting inserts have a considerably greater hardness and better hot hardness. Therefore, the ceramic cutting inserts usually have a long service life and can operate at extraordinary high temperatures, as well as with a very high cutting capacity. However, the ceramic cutting inserts are fairly brittle such that the compression strength is considerably greater than the tensile strength. Furthermore, the surfaces of ceramic cutting inserts are difficult to grind, in particular surfaces having irregular or complex shapes. This limits the tool designer's latitude to decide the geometry of the cutting inserts. Therefore, ceramic cutting inserts have, without exception, a fairly simple basic shape, which mostly has planar and/or rotationally symmetrical surfaces.
As to the geometry of the chip-removing edge of ceramic cutting inserts, two main categories can be distinguished, viz. chamfer-reinforced edges and rounded, so-called ER-edges (ER=Edge Radius), each one of which has its advantages and disadvantages. Rounded edges of the ER-type are found on cutting inserts, the edge of which is formed simply by the transition between a ground top surface and a ground clearance surface at an angle to the same, the edge being given the rounded shape thereof by a simple finishing, such as brushing. Such cutting inserts have the advantage of being comparatively easy-cutting, i.e., exposed only to moderate cutting forces, but the disadvantage of being mechanically weak and thereby inclined to become damaged or break (and therefore have an unpredictable service life). The edge of chamfer-reinforced cutting inserts is provided by grinding a slender chamfer surface in the transition between the top surface and the clearance surface, such that the chamfer surface forms an angle with the top surface as well as the clearance surface. For reasons of grinding technique, this angle (which is defined as the angle between the chamfer surface and an imaginary plane in the extension of the top surface) is traditionally in the range of 15-30°. Most commonly, the angle is approx. 20°. In such a way, a mechanically comparatively strong edge is provided, the resultant of the cutting forces that act against the chamfer surface being directed inward/downward in the cutting insert. Generally, the edge becomes more easy-cutting the smaller the above-mentioned angles are. However, a disadvantage of chamfer-reinforced cutting inserts is that the cutting forces become great, i.e., the cutting insert becomes blunt-cutting.
The fact that the two edge types have so very different properties in the mentioned respects causes problems for the users, in that it is difficult to predetermine which type of cutting inserts should be chosen to give optimal results for a given machining operation. Therefore, there is a need for ceramic cutting inserts, the strength and robustness of which is better than that of the ER-inserts, and the cutting performance of which simultaneously is better than that of the chamfer-reinforced cutting inserts.
U.S. Pat. No. 4,963,061 discloses a ceramic cutting insert that is formed with a chamfer surface adjacent to a clearance surface as well as a subsiding chip-breaking surface, which extends the chamfer surface and a flat upper side of the cutting insert. In this case, however, the chamfer surface has a width that is merely a fraction of the width of the chip-breaking surface, the chamfer surface serving solely as a narrow edge strengthening in the vicinity of a conventional chip breaker. U.S. Pat. No. 4,963,061 does not deal with the technical problem of providing distinct borderlines between a chamfer surface and an upper surface in connection with the grinding of the cutting insert and thereby providing a repeatable shape of the individual cutting inserts.
The present invention aims at providing for the above-mentioned needs and at providing a ceramic cutting insert that is more universally useful than previously known ceramic cutting inserts. Therefore, a primary object of the invention is to provide a ceramic cutting insert that combines the advantages of the ER-inserts and of the chamfer-reinforced cutting inserts, without having the disadvantages thereof. In other words, the edges of the cutting inserts should be both fairly strong and fairly easy-cutting.
Another object of the invention is to provide a ceramic cutting insert that can be made with more complex geometries than previously known ceramic cutting inserts.
Yet another object of the invention is to provide a ceramic cutting insert that has a repeatable exterior even if it is made with geometrically irregular surfaces, such that the border lines between the different surfaces of the cutting inserts always are similar and situated at the same locations, irrespective of variations in manufacture, to give a confident impression on the operator.